A known process for manufacture of the mentioned products involves a plurality of molds with a laying surface for latex foam material and a plurality of protuberances useful to the formation of recesses.
Each mold includes a cover through which foam emulsified with air and devoid of swelling media is injected until complete filling in of the mold.
Each mold is placed on a carrier and all the carriers separated from each other are moved through a tunnel vulcanizer which is steam heated.
At the end of the manufacturing process each single mold is open and the product with recesses is removed.
The said process enables to manufacture single products comprising recesses of limited length and width, ready for the required use.
Further processes are known, different from the above mentioned ones, for enabling the continuous manufacture of products of unlimited length comprising recesses, then cut to single sized articles.
One of these processes, described in U.S. Pat. No. 2,757,415, is based on continuous moving of a metal sheet provided with a plurality of perforations aimed to receive the protuberances located on the surface of a flexible belt. The metal sheet is ring-wound around two pulleys.
The belt is inside with respect to the metal sheet and ring-wound around two wheels having shorter diameter than the pulleys around which the sheet is wound.
The sheet and the belt are simultaneously moved in line towards the vulcanizer, in such a way that the protuberances of the belt pass through the perforations of the sheet penetrating through the latex foam material injected on the surface of the sheet at a suitable station.
The pulley on which the metal sheet moves downstream of the vulcanizer, being of greater diameter than the corresponding wheel of the belt, makes the removal of the product with recesses easier.
U.S. Pat. No. 5,229,138 describes in turn a continuous process for manufacture of products with recesses including a plurality of plates placed side by side to each other, the plates are joined at their sides to the links of two chains ring-wound around two gear wheels, respectively driving and driven.
Such plates are envisaged to form laying surfaces of the latex foam material injected on them at a first station, each plate includes a plurality of protuberances aimed at forming the recesses of the product.
A tunnel vulcanizer is placed between the injecting station and a removing station of the product with recesses; the removal of the product is carried out by means of two rolls rotating in opposite directions to each other, between which the product with recesses is dragged.
The plates form a ring shaped surface uninterrupted through its whole extension both in the upper and in the lower branch of the plant, as well as in the ring shaped stretches connecting the two branches around the two gear wheels.
The foam filled plates are moved forward through the vulcanizer and, after the removing of the product with recesses, come back, always with uninterrupted movement and side by side to each other, to the injecting station.
Such being the mentioned state of art, the applicant has posed the problem of obtaining a continuous manufacture of latex foam products comprising recesses with a considerable reduction of times spent both for maintenance operations and for the possible changes of parts of the apparatus which may be required to modify the features of the products to be manufactured, as well as a cutback of manufacturing costs, in no case jeopardizing the quality of products, but rather improving them.
It was thought then that the solution of the problem could come from employing a lower number of means than in the known technique. It has been perceived, in fact, that the known manufacturing techniques of products comprising recesses make use of a higher number of means than needed even if latex foam material has to cover only the stretch interposed between the injecting and the removing station, either a continuous foam laying surface made by a thin, ring-wound sheet, as described in U.S. Pat. No. 2,757,415, or a plurality of plates, placed side by side with no interruptions in a ring-like configuration, are generally used. As a matter of act, in both cases, the actual extension of the laying surface of foam material is active just in a limited part of its extension.
More particularly, it was thought that the problem could be solved excluding the known technique described in U.S. Pat. No. 2,757,415, since a continuous laying surface of foam material such as the one of a belt or a like, rolled up around two pulleys, could not be reduced. It has been moreover thought that the known solution required in any case a simultaneous perfect alignment between belt perforations and metal sheet protuberances very difficult to be maintained or a long time.
It was thought then that a solution could be found more generally starting from the hypothesis of adopting a number of laying surfaces continuous at any moment in a first stretch between the foam laying station and the removing station of the product with recesses, and discontinuous in the return stretch of the laying surfaces devoid of foam material towards the injecting station.
However not even this perception could solve the problem.